1. Field of the Invention
The present invention relates to an antenna device which integrates an antenna section with a medium for generating or detecting electromagnetic waves.
More specifically, the present invention relates to an antenna device which generates or detects a high-frequency electromagnetic wave (referred to as a terahertz wave in this specification) having an arbitrary band in the range between 30 GHz and 30 THz.
2. Description of the Related Art
In recent years, nondestructive inspection technologies have been developed which use terahertz waves. It is known that absorption lines of various materials including biomolecules are present in the frequency region of terahertz waves. An imaging technique for performing safe radiological examination without using X-rays is known in the field of application of electromagnetic waves of this frequency region. A spectroscopic technique for examining a bonding state of molecules by measuring an absorption spectrum or a complex permittivity in the inner part of a material is also known. In addition, techniques for analyzing biomolecules and techniques for evaluating the density or mobility of carriers are expected.
For developing these techniques, a technique of generating and detecting a terahertz wave is important. As such a technique, it is reported that a device for generating and detecting a terahertz wave was produced by patterning a flat-type antenna pattern and a fine gap on a semiconductor substrate (see Appl. Opt., Vol. 36, No. 30, pp. 7853-7859 (1997)). The device provides a terahertz wave by exciting a carrier in a gap by using an ultrashort pulsed-laser (femtosecond laser, for example) and accelerating the carrier by using an electric field separately applied to the gap. The structure of the device is adapted for operating as a detecting device as well.
Further, there is a technique for generating a terahertz wave by using a semiconductor technology. For instance, techniques using a gain medium such as a Gunn diode and a resonant tunneling diode (RTD) are known. Generators employing these gain mediums will constitute an oscillating circuit containing these gain mediums and properly regulate a load resistance and a phase in a desired frequency region to realize an oscillation state.
Conventionally, an electromagnetic wave obtained in this way is often radiated to the outside through a radiation device such as an antenna to which the generator has been connected. However, in the case of an electromagnetic wave in a high-frequency region, it is difficult to efficiently radiate the electromagnetic wave to the outside due to a propagation loss of the electromagnetic wave and a mismatch occurring between individually designed devices. For this reason, it is tried to regard an antenna device as one part of the load resistance constituting an oscillating circuit and integrate the antenna device into a monolithic structure (see IEEE Transaction on Microwave Theory Tech., vol. 42, pp. 734-741, 1994).
The antenna oscillator proposed in IEEE Transaction on Microwave Theory Tech., vol. 42, pp. 734-741, 1994 is of a transmission line or micro-strip line (MSL) type. The antenna oscillator has a patch antenna connected to a Gunn diode which is formed in the thickness direction of a dielectric film constituting a transmission line. The antenna oscillator uses the patch antenna as one part of the load resistance so as to satisfy the oscillation-starting condition expressed by the expressions (1) and (2) given below. The antenna oscillator also employs a stub for adjusting the phase in order to satisfy the phase condition. In addition, the patch antenna and another transmission line constituting a circuit are connected by an impedance converter circuit. As a whole, the present antenna oscillator has a configuration in which the components are integrated into a planar state.    (1) Real part of admittance: Re [Yact+Yload]<0    (2) Imaginary part of admittance: Im[Yact+Yload]=0
In the above respective expressions (1) and (2) specifying the conditions for gain and phase, Yact and Yload correspond to the admittance of a gain device (Gunn diode) and the admittance of a transmission line type oscillating circuit including the antenna, respectively.